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Date: Sat, 14 Nov 2015 21:18:20 +0200
From: "Michael S. Tsirkin" <mst@redhat.com>
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References: <002301d11df5$9d440a10$d7cc1e30$@samsung.com>
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Subject: Re: [Qemu-devel] [PATCH] vhost: Fix aborting if KVM does not
	support eventfds
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To: Pavel Fedin <p.fedin@samsung.com>
Cc: qemu-devel@nongnu.org

On Fri, Nov 13, 2015 at 12:28:10PM +0300, Pavel Fedin wrote:
> If you happen to have a stock kernel of old version, like 3.x, and you
> attempt to enable vhost by setting vhost=on, qemu aborts with error:
> 
> kvm_mem_ioeventfd_add: error adding ioeventfd: Function not implemented
> 
> This patch adds capability check, so that vhost gets disabled instead. A
> warning is displayed, explaining the reason.
> 
> This problem can be observed with libvirt, which checks for /dev/vhost-net
> availability and just inserts "vhost=on" automatically in this case.
> 
> Signed-off-by: Pavel Fedin <p.fedin@samsung.com>

How come you have /dev/vhost-net though?
That was supposed to only be there if you have vhost-net,
and that never existed on kernels without eventfd.

> ---
>  hw/virtio/vhost.c | 6 ++++++
>  1 file changed, 6 insertions(+)
> 
> diff --git a/hw/virtio/vhost.c b/hw/virtio/vhost.c
> index 1794f0d..3121e19 100644
> --- a/hw/virtio/vhost.c
> +++ b/hw/virtio/vhost.c
> @@ -24,6 +24,7 @@
>  #include "hw/virtio/virtio-bus.h"
>  #include "hw/virtio/virtio-access.h"
>  #include "migration/migration.h"
> +#include "sysemu/kvm.h"
>  
>  static struct vhost_log *vhost_log;
>  static struct vhost_log *vhost_log_shm;
> @@ -1083,6 +1084,11 @@ int vhost_dev_enable_notifiers(struct vhost_dev *hdev, VirtIODevice *vdev)
>          r = -ENOSYS;
>          goto fail;
>      }
> +    if (!kvm_eventfds_enabled()) {
> +        error_report("KVM does not support MMIO eventfds");
> +        r = -ENOSYS;
> +        goto fail;
> +    }
>  
>      for (i = 0; i < hdev->nvqs; ++i) {
>          r = k->set_host_notifier(qbus->parent, hdev->vq_index + i, true);
> -- 
> 1.9.5.msysgit.0
>